For the purpose of fracture surgery, a bone plate system is configured to press fractured bone fragments against each other and to pull bone fragments into contact with each other, thereby promoting treatment thereof. The bone plate system includes a bone plate and bone screws. The bone plate may have a hole in which an internal thread is formed and a hole in which an internal thread is not formed. The bone screws include a locking screw having a head on which an external thread is formed and a non-locking screw having a head on which an external thread is not formed. Each of the locking screw and the non-locking screw is provided with a shank on which an external thread to be coupled to a bone is formed.
The external thread formed on the head of the locking screw is coupled to the internal thread of the bone plate, thereby fixing an angle at which the shank is coupled to a bone. The locking screw is coupled so that the center axis thereof is coaxial with the center axis of the internal thread. Furthermore, the locking screw can prevent itself from being untightened or loosened by a dynamic load acting on the bone fragments and the bone plate after a surgical operation. However, the locking screw has a limited bone fragment pressing ability which affects a fracture treatment.
The external thread of the non-locking screw may be coupled to a bone at different angles. However, the non-locking screw is not fixed to the bone plate because an external thread is not formed in the head thereof. Thus, during or after a surgical operation, the angle between the bone plate and the non-locking screw may be changed. Furthermore, the non-locking screw may be untightened or loosened by a dynamic load acting on the bone fragments and the bone plate after a surgical operation. Thus may lead to an insufficient treatment. That is to say, the locking screw is capable of fixing a bonding angle of bone fragments but is limited in its insertion angle and its pressing ability. The non-locking screw is capable of being inserted into a bone at different desired angles to press bone fragments but has a drawback in that the non-locking screw may be loosened.
Korean Patent No. 10-1471627 entitled “bone plate system” and Korean Patent Application Publication No. 10-2014-0144564 entitled “bone plate system”, both of which are filed by the assignee of the subject application, disclose a bone plate and a locking screw. The bone plate has a lower surface facing a bone, an upper surface, and an internal thread formed in a hole extending from the upper surface to the lower surface. The locking screw includes a head and a shank extending from the head in an axial direction and having an external thread to be coupled to a bone. The head has an external thread formed in a tapered portion whose diameter decreases along an axial direction. The external thread of the head is tightened to the internal thread of the bone plate so that the angle of the locking screw can be adjusted. The disclosures of the patent documents cited above are incorporated herein by reference.
In the bone plate systems mentioned above, a strong fixing force is required in order to stably fix a bone. Moreover, it is required that the fixing force generated by tightening a screw can be transmitted to a bone via the bone plate. Accordingly, a demand has existed for a bone plate system which can generate a strong fixing force during fracture surgery and which can be easily used by a surgeon.